The present invention is directed to a press-out piston for dispensing a flowable plastic substance from a cylindrical container with the piston having an annular groove in its outer circumferential surface.
Containers with press-out pistons for forcing a plastic substance out of a container are usually used only once and simultaneously serve as a packing and, together with a corresponding pressure applying device, as a dispensing apparatus for determined amounts of the substance. For reasons of cost, the container and the press-out piston are produced from plastics material. Considerable force is required, based on the viscosity of the substance being dispensed, for forcing the substance out of the container. Accordingly, high pressure is exerted by the substance on the piston and also on the container wall. Due to the pressure acting on the container wall, the container tends to expand radially so that a certain amount of radial play develops between the press-out piston and the container. Such radial play can form an annular gap through which a part of the substance being dispensed can escape out of the rear end of the container opposite to the direction in which the substance is dispensed. If such substance leaks out of the rear end of the container it can interfere with the proper dispensing of the substance and can result in interruptions in the dispensing operation for short or long periods of time.
In known press-out pistons a variety of means have been used to prevent the substance from escaping out of the container, for instance, radially resilient sealing lips, pressure compensation grooves and the like have been utilized. Other press-out pistons have been widened radially through the deformation of the piston head. These various means have been either too costly or have not provided a sufficient sealing effect.